The term “green chemistry” has been used to describe synthesis of chemicals from biorenewable feed stocks. It is considered a sustainable alternative to petroleum-based chemistry, and may mitigate the effect of high oil prices. One of the challenges facing transition to a green chemical industry is that the existing production facilities and infrastructure are designed around hydrocarbon feed stocks. For example, the building blocks of the chemical industry, olefins and BTX aromatics (benzene, toluene, and xylene), are produced in steam crackers (also referred to as ethylene crackers) and catalytic reformers that run on light hydrocarbons. Synthesis gas, or syngas for short, is another chemical building block. Syngas is a mixture of carbon monoxide and hydrogen and can be produced by steam reforming of light hydrocarbons. Naphtha is a C5-C9 hydrocarbon cut that is commonly used as the feedstock for both catalytic reformers and steam crackers. It may also be used as feed for steam reforming units for production of syngas.
In particular, paraffinic naphtha is considered a highly desired feed for steam crackers due to its high ethylene and propylene yields. Production of naphtha from biorenewable sources would enable transition to a green chemical industry without the need to develop new chemistries and build new production facilities.
Naphtha is also a common industrial solvent. Extraction of lipids from seeds (e.g. soybean oil) uses n-hexane from petroleum naphtha. However virgin naphtha distilled from crude oil typically contains 2.5% to 5.0% by weight benzene, a well-known carcinogen. As such, multiple and costly purification steps (e.g. solvent extraction and super-fractionation) are required before food-contact quality hexane is obtained.
The term “sustainable energy” has been used to refer to renewable sources of energy. Biorenewable fuels are a key component of sustainable energy initiatives. Naphtha may be used directly as fuel in industrial furnaces or turbines. It may also be used in small industrial engines such as lawnmowers and chainsaws.
Furthermore, since naphtha hydrocarbons are in the motor gasoline boiling range, they may be used as a gasoline blend stock. Gasoline blends need to meet vapor pressure and octane rating requirements.
One method of producing naphtha hydrocarbons from biomass is by the Fischer-Tropsch (F-T) process. U.S. Pat. No. 7,214,720 to Bayle and co-inventors describes a process involving the steps of (a) gasification, (b) syngas purification, (c) F-T conversion, (d) separation, and (e) recycle of at least a portion of the naphtha to gasifier. Although the process produces the desired hydrocarbon naphtha from a bio-renewable source, gasifiers suffer from a low reliability record. Additionally, the capital costs associated with gasification and F-T conversion are known to be very high. This is in part due to solids handling requirements for the gasifier and heat removal provisions for the highly exothermic F-T reaction. Furthermore, the need to recycle part of the naphtha to the gasifier further reduces the efficiency of this process as a source of bio-renewable naphtha.
U.S. Pat. No. 5,186,722 to Cantrell and Chong describes a catalytic process to convert biomass feeds such as limonene to a composition of cyclic and aromatic compounds in the naphtha boiling range. Although these products are reported to have high octane rating and hence good gasoline blend stocks, they lack the desired properties as a chemical feed stock. As feeds for steam crackers, aromatic compounds give low ethylene and propylene yields. More importantly, the terpene feeds used in the process are among the only biomass sources that to begin with are hydrocarbons. Virtually all other sources of biomass have high oxygen content, typically greater than 10 wt %.
Deoxygenation of biomass feeds such as triglycerides and fatty acids are disclosed in U.S. Pat. No. 7,232,935 to Jakkula and co-inventors. A two step process is disclosed which includes hydrodeoxygenation of triglycerides/fatty acids followed by hydroisomerization. The process produces diesel boiling-range isoparaffins.
To this end, there is a need for biorenewable naphtha that can be used as feed stock for existing petrochemical and refining facilities. In particular, the present invention is a method process for converting high oxygen content biomass, such as sources of triglycerides and/or fatty acids, into naphtha boiling-range hydrocarbons using standard refining processes.